Quantum information operations may be performed by assigning logical states to the quantum states in physical systems. The logical states are analogous to the zero and one states for a conventional digital computer. Quantum information can be encoded and manipulated using single photon states. Many in principle demonstrations of quantum information tasks have now been accomplished in single photon optics including quantum cryptography [1], quantum dense coding [2] and quantum teleportation [3]. More recently two qubit gates have been realised [4,5] using conditional techniques [6]. Such experiments typically make use of polarisation to encode the qubits.
The tools used in polarisation encoding schemes are principally the half-wave plate which is used to make arbitrary rotations of a state in the polarisation basis, the polarising beamsplitter which is used to separate (or combine) photons into (from) different spatial and polarisation modes and the quarter-wave plate which is used to introduce relative phase shifts between the two bases.
It will be appreciated that polarisation is not the only photonic degree of freedom available to the experimentalist. For example schemes in which the temporal [7] or spatial [8] optical modes form the quantum basis have also been realised. Here we consider a basis consisting of two frequency optical modes.